A liquid crystal display comprises two transparent substrates, for example, glass substrates, provided with transparent electrodes and disposed with a gap of the order of 1 .mu.m to 10 .mu.m provided therebetween, and a liquid crystal material sealed in the gap, wherein the liquid crystal is orientated in a predetermined direction by application of a voltage between the electrodes, thereby forming transparent and opaque portions, and thus displaying an image. In a color liquid crystal display, a color filter for three colors, i.e., red (R), green (G) and blue (B), corresponding to the three primary colors of light are provided over either of the transparent electrode substrates to effect additive color mixture of the three primary colors by the shutter action of the liquid crystal, thereby displaying a desired color.
Such a color filter for a color liquid crystal display comprises a transparent substrate, a colored layer, a protective film, and a transparent electrically conductive film, which are stacked in the mentioned order. The color filter is disposed to face another transparent substrate, which has electrodes or thin-film transistors formed in opposing relation to the colored pixels of the three primary colors, i.e., R, G and B, with a gap of several .mu.m held therebetween, and a liquid crystal substance is sealed in the gap, thereby forming a liquid crystal display.
FIG. 1 is a sectional view of one example of the color liquid crystal display. The color liquid crystal display 1 includes a color filter 2 and an opposing substrate 3 formed with thin-film transistors (TFT) or transparent electrodes. The color filter 2 and the substrate 3 are disposed to face each other across a predetermined gap and bonded together by using a sealing medium 4 formed by mixing reinforcing fibers with an epoxy resin material or the like. A liquid crystal 5 is sealed in the space defined between the color filter 2 and the TFT substrate 3.
The color filter 2 will be explained below more specifically. A substrate 6, for example, a glass substrate, has a black matrix 7 formed thereon so as to divide adjacent colored pixels by using a metal, e.g., chromium, or a resin material colored with a dye or a pigment, thus forming a colored layer comprising colored pixels of the three primary colors, that is, red colored pixels 8, green colored pixels 9, and blue colored pixels 10, which are divided from each other by the black matrix 7. In addition, a protective film 11 is provided over the colored layer to protect it, and a transparent electrode film 12 for driving the liquid crystal is provided over the protective film 11. Further, an orientation layer 13 for orientating the liquid crystal is formed over the transparent electrode film 12.
Hitherto, colored pixels provided on the color filter are formed as follows. A transparent substrate, for example, a glass substrate, is coated with a transparent resin material obtained by adding a photosensitive material, e.g., a dichromate, a chromate, or a diazocompound, to a hydrophilic resin material, e.g., polyvinyl alcohol, polyvinyl pyrrolidone, gelatin, casein, or glue, thereby forming a transparent photosensitive resin layer. Then, a photomask having an opening pattern with a predetermined configuration is placed over the transparent resin layer, and exposure and development are carried out to form a first resin layer, which is then dyed with a desired dye to form first transparent colored pixels. Next, a transparent hydrophobic resin film for preventing dyeing is formed over the first transparent colored pixels in order to prevent migration of the dye. Thereafter, second transparent colored pixels are formed in the same way as in the case of the first transparent colored pixels. By repeating the above-described process, transparent colored pixels of at least two or three different colors are formed on the substrate.
However, the above-described method requires formation of a transparent resin film for preventing dyeing for each color in order to provide transparent colored pixels of a plurality of colors and hence suffers from the disadvantage that the production process is extremely complicated.
In addition, the color filter for a color liquid crystal display has a transparent electrode film formed thereon for driving the liquid crystal. The transparent electrode film is formed mainly by using an ITO film that is a composite oxide comprising indium oxide and tin oxide, which has excellent characteristics. However, since the electrical resistance of the ITO film depends on the film forming temperature, it is necessary in order to obtain an ITO film of low resistance to carry out the film forming process by heating at a temperature of about 300.degree. C. However, the conventional color film having transparent pixels colored by using a dye has a probability that the dye-colored layer will be caused to change color by heat applied during a manufacturing process, e.g., the ITO film formation, or light externally applied during use. Thus, the prior art is limited in the heat resistance and the light resistance, so that the conventional color liquid crystal display equipped with such a color filter is not satisfactory.
It may be considered using a thermosetting resin material which endures the heat treatment in the manufacturing process as a protective film for the color filter. However, in the case of a thermosetting resin material, the protective film is formed over the whole surface of the color filter and it is difficult to limit the area where the protective film is formed to a predetermined region.
If the protective film is formed over the whole surface of the color filter, the color filter and the opposing substrate are bonded by a sealing medium with the protective film interposed therebetween, so that no adequate bond strength can be obtained. In a case where the color filter is judged to be defective in display quality on inspection after the color filter and the opposing substrate have been bonded together and the sealing medium and the tab are successively separated in order to reuse the color filter, if the colored layer has a protective layer on the outer peripheral portion thereof, the transparent electrode film formed on the color filter is undesirably separated together with the sealing medium or the tab, resulting in a failure to reuse the color filter.
Under these circumstances, it has also been a conventional practice to employ a photo-setting resin material which enables a region where it is set to be readily limited by using a photomask, in order to limit the area where the protective film is formed to a specific region. However, a photosensitive polyimide resin material which has heretofore been used as a photo-setting resin material is highly hygroscopic, inferior in the resistance to corrosion from an alkaline solution used in the formation of electrodes or other process and costly and hence undesirable for practical use. In addition, a photosensitive acrylic resin material which has also heretofore been used is inferior in the resistance to heat and also involves the problem that cracks or wrinkles may grow on the color filter when a relatively thick transparent electrically conductive film is formed on the protective film with a view to lowering the electrical resistance.
If the surface of the protective film is not even, the transparent electrode film becomes uneven, so that the gap between the color filter and the opposing substrate varies locally, resulting in differences in the optical rotatory power of the liquid crystal. Thus, unevenness of display results.
Further, if a photo-setting resin material is set by irradiation with light, e.g., ultraviolet light, by using a photomask, it is possible to limit the area where the protective film is formed. In such a case, however, since the protective film is formed in a pattern faithful to the pattern of the photomask, a step corresponding to the thickness of the protective film is produced at the peripheral edge portion of the protective film set. If a transparent electrode film for driving the liquid crystal is formed over such a protective film, it extends as far as the peripheral portion of the substrate beyond the area where the protective film is formed, so that the transparent electrode film is not so thick at the step portion as it is at the even portion. Thus, a problem in terms of strength arises. In particular, when the transparent electrode film is etched in patterns corresponding to a large number of pixels, side etching progresses particularly at the step portion, so that there is a possibility of disconnection of the transparent electrode film.
It is an object of the present invention to provide a color filter having a colored layer comprising colored pixels, which is excellent in both heat resistance and light resistance, and a protective film which is superior in heat resistance, satisfactorily hard and excellent in adhesion to the substrate and which has a gentle step at the peripheral portion of the substrate, and also provide a method of producing the color filter.